In order to monitor the quality of air, filter cassettes are used for sampling airborne substances, i.e. substances that are in suspension in the air. Typically, a filter cassette comprises a conduit having an air inlet and an air outlet, with a filter being provided therebetween. As air is brought to flow in the conduit from the inlet to the outlet, a portion of the airborne substances comprised in the air are captured by the filter. By analyzing the content of the filter, knowing an air flow and a time exposure, it is possible to make an assessment of the airborne substance concentration.
Problem Description
Two- and three-piece plastic cassettes are commonly used for collecting personal samples of airborne solid particles and/or liquid droplets and/or gas. The cassettes have many applications such as air quality monitoring in work places as well as environmental analyses. The accuracy of related sampling methods is of prime importance.
The cassettes, however, are prone to external leaks and bypass leakage, notably if they are not properly assembled.
A known filter cassette 10 is illustrated in FIG. 1. The filter cassette 10 comprises a cap 12 and a base 14. The cap 12 has an inlet 16 for conducting air into filter cassette 10; similarly, the base 14 has an outlet 18 for conducting air from inside the filter cassette 10 to the outside. A filter 20 and a backup pad 22 are interposed between cap 12 and base 14, the cap 12 being atop base 14. Base 14 is so configured as to accommodate cap 12. When cap 12 and base 14 are assembled, filter 20 and backup pad 22 are compressed therebetween, thus forming a first chamber 24 upstream of filter 20, and a second chamber 26 downstream of backup pad 22. Chamber 24 is characterized by a pressure P1 and an airflow F1 entering thereinto via inlet 16; similarly, chamber 26 is characterized by a pressure P2 and an airflow F2 exiting therefrom at 18.
FIG. 1 illustrates an external leak 28, which refers to air flowing from inside filter cassette 10 to the outside thereof. The external leak 28 occurs between cap 12 and base 14. In a three-piece cassette (not shown), which also comprises a ring, an external leak can occur between the cap and the ring, and/or the ring and a base.
FIG. 1 also illustrates a bypass leakage 30, which refers to air 32, from inlet 16, which flows around filter 20 and around or through backup pad 22. In this manner, only a portion of air 32 goes through the filter 20. However, all air will exit filter cassette 10 through outlet 18. Bypass leakage 30 can occur because of improper assembly, for example because of insufficient assembly force or because of misalignment [1].
Both leaks and bypass leakages can result in a loss of airborne substances that should have been collected by filter 20, which in turn reflects in underestimating the concentration thereof. A poorly assembled cassette typically leads to a more than 1% loss of airborne substances [2].
Conventional Testing Methods
To ensure that a filter cassette is properly assembled, the cassette can be tested by using a particle counter for evaluating a number of particles before the filter (inlet) and after the filter (outlet) [3, 4 and 5]. This method, however, does not work well with a fibrous filter, where fibers released therefrom can cause an overestimation (as much as 20-fold) of a leak rate. In addition, air could be leaking while particles that can be detected do not leak, for example because of shape and size of the particles and size of leak.
A second method, in reference with FIGS. 2a and 2b, consists in evaluating a pressure stability in filter cassette 10 after drawing vacuum into it [6] by using a vacuum pump 32. A pressure gauge 34 is mounted on inlet 16, the latter being obstructed. A vacuum is drawn from the outlet 18 by using vacuum pump 32, while a valve 36 interposed between vacuum pump 32 and outlet 18 is open (FIG. 2a). Valve 36 is then closed (FIG. 2b). If P1 does not stay equal to P2, then there is an indication that there is an external leak. This second method, however, cannot be used for determining whether there is a bypass leakage around filter 20.
A third method, which is an alternative to the second method but nevertheless cannot detect a bypass leakage, consists in measuring flows instead of pressures. As illustrated in FIG. 3, a flow F1 about inlet 16 and a flow F2 about outlet 18 are induced, for example by using vacuum pump 32. A first flow meter 38 is provided just before inlet 16; similarly, a second flow meter 40 is provided just after outlet 18. If there is no external leak, F1 equals F2; otherwise, there is an indication of the presence of an external leak.